1. Field of the Invention
This invention relates to an electrode structure in a discharge tube having opposing electrodes which are sealed in a housing.
2. Description of the Prior Art
A discharge tube having two electrodes opposed to each other within a hermetically sealed housing is popularly known and used as an electric part for providing a certain voltage.
A prior art discharge tube is now described referring to FIG. 1. As shown in FIG. 1, a discharge tube generally includes a hermetically sealed housing 103, a pair of electrodes 101, 102 which are opposed to each other within the hermetically sealed housing 103. An inert gas 105 is sealed in the space within the hermetically sealed housing 103. When a high electric voltage is applied to the electrodes 101, 102, a spark discharge X occurs between the opposing and portions 101a and 102a of the electrodes 101 and 102. Most of the prior art discharge tubes have electrodes formed in a simple rod shape as shown in FIG 1. The ends of the opposing portions 101a and 102a are exposed to the inner walls of the housing 103.
When spark discharge occurs repeatedly in the prior art discharge tube, electrode material is evaporated by the sparks scatters around the electrodes 101 and 102 and to adhere to the inner walls of the housing 103. As a result, the electrode material is spattered to form a film 106 throughout the entire surface of the inner walls of the housing 103. Since the film 106 is electrically conductive, there is a so-called creeping discharge Y or the discharge between the electrodes 101 and 102 which takes place via the film 106 takes place. Such a creeping discharge lowers the discharge starting voltage and makes the discharge operation unstable. It is therefore impossible to obtain a predetermined level of voltage.